In a technology disclosed in Japanese Unexamined Patent Application Publication No. 2002-367098, an allowable speed of each curve is calculated (Step S4) if multiple continuous curves exist (YES in Step S3 in FIG. 3) and a minimum allowable speed, among the allowable speeds calculated for the respective curves, is set as a target speed Vs while an own vehicle is traveling along the curves (Steps S4 to S6). If the current vehicle speed is higher than the target speed Vs (YES in Step S10 in FIG. 3), when a distance L required for retarding the vehicle to the target speed Vs which is calculated from the target speed Vs is longer than or equal to a retardation allowance distance LCA to the entrance of the curve (YES in Step S11) the target speed Vs is alerted to a driver in order to prompt the driver to perform a retardation operation before the own vehicle enters the nearest curve (S12).
As described above, in Japanese Unexamined Patent Application Publication No. 2002-367098, the minimum allowable speed, among the allowable speeds calculated for the respective continuous curves, is set as the target speed Vs while the own vehicle is traveling along the curves (Steps S4 to S6). In other words, even if the multiple continuous curves exist, the target speed is alerted only before the first curve and the target speed is the minimum allowable speed in the multiple curves in Japanese Unexamined Patent Application Publication No. 2002-367098.
Accordingly, in Japanese Unexamined Patent Application Publication No. 2002-367098, it is necessary for the driver to predict information about the continuous curves in response to the one alert and it is difficult for the driver to estimate the timing when the operation for each curve is performed.